Flat-panel displays are of great interest as information displays for computing, entertainment, and communications. Electroluminescent (EL) flat-panel display technologies, such as organic light-emitting diode (OLED) technology provides benefits in color gamut, luminance, and power consumption over other technologies such as liquid-crystal display (LCD) and plasma display panel (PDP). However, EL displays suffer from performance degradation over time. In order to provide a high-quality image over the life of the display, this degradation must be compensated for.
EL displays typically comprise an array of identical subpixels. Each subpixel comprises a drive transistor (typically thin-film, a TFT) and an EL device, the organic diode that actually emits light. The light output of an EL device is roughly proportional to the current through the device, so the drive transistor is typically configured as a voltage-controlled current source responsive to a gate-to-source voltage Vgs. Source drivers similar to those used in LCD displays provide the control voltages to the drive transistors. Source drivers convert a desired code value step 74 into an analog voltage step 75 to control a drive transistor. The relationship between code value and voltage is typically non-linear, although linear source drivers with higher bit depths are becoming available. Although the nonlinear code value-to-voltage relationship has a different shape for OLEDs than the characteristic LCD S-shape (shown in e.g. U.S. Pat. No. 4,896,947), the source driver electronics required are very similar between the two technologies. In addition to the similarity between LCD and EL source drivers, LCD displays and EL displays are typically manufactured on the same substrate: amorphous silicon (a-Si), as taught e.g. by Tanaka et al. in U.S. Pat. No. 5,034,340. Amorphous Si is inexpensive and easy to process into large displays.
Degradation Modes
Amorphous silicon, however, is metastable: over time, as voltage bias is applied to the gate of an a-Si TFT, its threshold voltage (Vth) shifts, thus shifting its I-V curve (Kagan & Andry, ed. Thin-film Transistors. New York: Marcel Dekker, 2003. Sec. 3.5, pp. 121-131). Vth typically increases over time under forward bias, so over time, Vth shift will, on average, cause a display to dim.
In addition to a-Si TFT instability, modern EL devices have their own instabilities. For example, in OLED devices, over time, as current passes through an OLED device, its forward voltage (Voled) increases and its efficiency (typically measured in cd/A) decreases (Shinar, ed. Organic Light-Emitting Devices: a survey. New York: Springer-Verlag, 2004. Sec. 3.4, pp. 95-97). The loss of efficiency causes a display to dim on average over time, even when driven with a constant current. Additionally, in typical OLED display configurations, the OLED is attached to the source of the drive transistor. In this configuration, increases in Voled will increase the source voltage of the transistor, lowering Vgs and thus, the current through the OLED device (Ioled), and therefore causing dimming over time.
These three effects (Vth shift, OLED efficiency loss, and Voled rise) cause each individual OLED subpixel to lose luminance over time at a rate proportional to the current passing through that OLED device. (Vth shift is the primary effect, Voled shift the secondary effect, and OLED efficiency loss the tertiary effect.) Therefore, as the display dims over time, those subpixels that are driven with more current will fade faster. This differential aging causes objectionable visible burn-in on displays. Differential aging is an increasing problem today as, for example, more and more broadcasters continuously superimpose their logos over their content in a fixed location. Typically, a logo is brighter than content around it, so the pixels in the logo age faster than the surrounding content, making a negative copy of the logo visible when watching content not containing the logo. Since logos typically contain high-spatial-frequency content (e.g. the AT&T globe), one subpixel can be heavily aged while an adjacent subpixel is only lightly aged. Therefore, each subpixel must be independently compensated for aging to eliminate objectionable visible burn-in.
Prior Art
It has been known to compensate for one or more of these three effects. Considering Vth shift, the primary effect and one which is reversible with applied bias (Mohan et al., “Stability issues in digital circuits in amorphous silicon technology,” Electrical and Computer Engineering, 2001, Vol. 1, pp. 583-588), compensation schemes are generally divided into four groups: in-pixel compensation, in-pixel measurement, in-panel measurement, and reverse bias.
In-pixel Vth compensation schemes add additional circuitry to each subpixel to compensate for the Vth shift as it happens. For example, Lee et al., in “A New a-Si:H TFT Pixel Design Compensating Threshold Voltage Degradation of TFT and OLED”, SID 2004 Digest, pp. 264-274, teach a seven-transistor, one-capacitor (7T1C) subpixel circuit which compensates for Vth shift by storing the Vth of each subpixel on that subpixel's storage capacitor before applying the desired data voltage. Methods such as this compensate for Vth shift, but they cannot compensate for Voled rise or OLED efficiency loss. These methods require increased subpixel complexity and increased subpixel electronics size compared to the conventional 2T1C voltage-drive subpixel circuit. Increased subpixel complexity reduces yield, because the finer features required are more vulnerable to fabrication errors. Particularly in typical bottom-emitting configurations, increased total size of the subpixel electronics increases power consumption because it reduces the aperture ratio, the percentage of each subpixel which emits light. Light emission of an OLED is proportional to area at a fixed current, so an OLED device with a smaller aperture ratio requires more current to produce the same luminance as an OLED with a larger aperture ratio. Additionally, higher currents in smaller areas increase current density in the OLED device, which accelerates Voled rise and OLED efficiency loss.
In-pixel measurement Vth compensation schemes add additional circuitry to each subpixel to allow values representative of Vth shift to be measured. Off-panel circuitry then processes the measurements and adjusts the drive of each subpixel to compensate for Vth shift. For example, Nathan et al., in US 2006/0273997(A1), teach a four-transistor pixel circuit which allows TFT degradation data to be measured as either current under given voltage conditions or voltage under given current conditions. Nara et al., in U.S. Pat. No. 7,199,602, teach adding an inspection interconnect to a display, and adding a switching transistor to each pixel of the display to connect it to the inspection interconnect. Kimura et al., in U.S. Pat. No. 6,518,962, teach adding correction TFTs to each pixel of a display to compensate for EL degradation. These methods share the disadvantages of in-pixel Vth compensation schemes, but some can additionally compensate for Voled shift or OLED efficiency loss.
Reverse-bias Vth compensation schemes use some form of reverse voltage bias to shift Vth back to some starting point. These methods cannot compensate for Voled rise or OLED efficiency loss. For example, Lo et al., in U.S. Pat. No. 7,116,058, teach modulating the reference voltage of the storage capacitor in an active-matrix pixel circuit to reverse-bias the drive transistor between each frame. Applying reverse-bias within or between frames prevents visible artifacts, but reduces duty cycle and thus peak brightness. Reverse-bias methods can compensate for the average Vth shift of the panel with less increase in power consumption than in-pixel compensation methods, but they require more complicated external power supplies, can require additional pixel circuitry or signal lines, and may not compensate individual subpixels that are more heavily faded than others.
Considering Voled shift and OLED efficiency loss, U.S. Pat. No. 6,995,519 by Arnold et al. is one example of a method that compensates for aging of an OLED device. This method assumes that the entire change in device luminance is caused by changes in the OLED emitter. However, when the drive transistors in the circuit are formed from a-Si, this assumption is not valid, as the threshold voltage of the transistors also changes with use. The method of Arnold will thus not provide complete compensation for subpixel aging in circuits wherein transistors show aging effects. Additionally, when methods such as reverse bias are used to mitigate a-Si transistor threshold voltage shifts, compensation of OLED efficiency loss can become unreliable without appropriate tracking/prediction of reverse bias effects, or a direct measurement of the OLED voltage change or transistor threshold voltage change.
Alternative methods for compensation measure the light output of each subpixel directly, as taught e.g. by Young et al. in U.S. Pat. No. 6,489,631. Such methods can compensate for changes in all three aging factors, but require either a very high-precision external light sensor, or integrated light sensors in each subpixel. An external light sensor adds to the cost and complexity of a device, while integrated light sensors increase subpixel complexity and electronics size, with attendant performance reductions.
Existing Vth compensation schemes are not without drawbacks, and few of them compensate for Voled rise or OLED efficiency loss. Those that compensate each subpixel for Vth shift do so at the cost of panel complexity and lower yield. There is a continuing need, therefore, for improving compensation to overcome these objections to compensate for EL panel degradation and prevent objectionable visible burn-in over the entire lifetime of an EL display panel.